Carburetors

[Thehunted]

Hi all, I've just joined. I wondered if I can get a bit of advice about these carbs which are driving me mad. 

 

I've decided to restore a Kawasaki GPX750r which has been sitting since 2011. It comes with a set of Keihin CV34 carbs which I've thoroughly cleaned inside and out. If I open the throttle quickly it bogs and stalls. If I slowly open it slowly it revs to about 5k then splutters and seems to stop revvingany higher. I think the pilot jets may still be partially blocked, but after restripping them I noticed the main jet is a 105, but the manual and a couple of sites online say the stock main jet is 112. It's had a 4 into 1 exhaust fitted at some point. Would this main jet size cause the issue I'm talking of. 

 

This is the first set of carbs I've worked and have to say I appreciate fuel injection alot more now .

 

Any help would be appreciated 

 

Thanks Mark 

[Tinkicker]

A four into one exhaust if less restrictive, will certainly cause it to run leaner. 

What market is your manual for?  Different markets often had different carb settings.  What is supplied in the US is often different to Europe or Oceana.

 

The only way to be clear is to have a manual specific to the UK model.

 

Does your carbs have accellerator pumps fitted? If so and they are faulty, that would cause the bogging as would restricted pilot jets.

Are the carb slides free and able to open to their full extent?  

 

It certainly sounds lean to me.  Of course, these are only generalisations.  A diagnosis is impossible over the internet.

Edited June 21, 2024 by Tinkicker

[JRH]

112 jets are for UK market, (@Tinkicker 118 in California are the only other jets).

I had a GPX750 many years ago and fitted a 4 into 1 exhaust after the factory system rotted away. Never changed the jets (at the time the manufacturer did say no jet change required). 
If mine was let standing for some time, usually over winter it would suffer a bit like that. Copious amounts of carb cleaner run through the system and Italian tune ups seemed to keep it running reasonably smoothly

 

Edited June 21, 2024 by JRH

[Thehunted]

Thanks for the reply both. I've only managed to get the Haynes manual which probably isn't the best to be working from. It's a jap import I believe, so not sure what the jets are for that. Its still restricted to 112mph. It doesn't have an accelerator pump as far as I know, nothing electrical to it anyway, just the usual vacuum hoses and fuel hose. I'll order some 112 jets and give the pilots another clean or maybe order some new ones too. 

 

Thanks 

Edited June 21, 2024 by Thehunted

[Hairsy]

Pilot jets can be cleaned but the holes are so small that it's really easy to leave a small bit of dirt in there. Also, you can clean them and then inadvertently leave a little bit of dirt in the float bowl or fuel hose that then immediately blocks them up again. Probably worth having another go at cleaning them.

 

Having the right size mains is definitely wise but your description of symptoms does suggest pilots would be a good place to focus.

 

An accelerator pump is a mechanical device - it's part of the carb and pushes extra fuel through when opening the throttle. Your manual should tell you whether you have them. No wires!

 

The other thing to check for is air leaks. Are the ports for carb balancing all blocked off? Are the carb rubbers a tight fit to the carb and the engine?

Edited June 21, 2024 by Hairsy

[Tinkicker]

#112 main jet.  Kawasaki part number 920631117 around £8.50 ea.

 

#35 pilot jet.  Kawasaki part number 920641101 £10.50 or thereabouts.

Applicable for European model GPX750R from 1987 to 89.

 

Do not be tempted by cheapo aftermarket jets.  Get genuine.  Often an aftermarket jet is not an identical jet to OEM.

 

Don't ask me how I know this... It was a very expensive lesson to learn..

 

 

Edited June 21, 2024 by Tinkicker

[Bianco2564]

Have you checked the diaphragm rubbers that they are not split?

Check carb to cyl head rubbers are not split or hardened.

Check carb balance with vac gauges.

If it’s a speed restricted import, are there restrictions anywhere, inlet or exh ports that explain the smaller mains?

Pretty sure accelerator pumps are not fitted to CV carbs, only on fixed Venturi carbs. Acceleration enrichment is controlled by the rate at which the piston or slide rises, this is controlled by the spring rate and in the case of a car SU carb, the oil damper in the piston.

[Ian Frog]

As an easy check to what @Bianco2564 correctly mentions I would always start the bike and have it ticking over if possible then spray carb cleaner over the rubbers connecting the carbs to the head.

An air leak will be revealed soon enough I promise.

@Tinkicker is perfectly correct saving on non original jets is a false economy and will bite you in the ars* don`t ask !

Good luck.

Cheers

Ian

[Thehunted]

Thanks again all. Much appreciated with your replies. All rubbers are fine. I've purchased new air box to carb rubbers which were a complete rip off. All diaphragm rubbers have been checked and I've replaced one which had been squashed between the cover and base of the card and was slightly torn. I believe the restriction is built into the dash clocks, but not 100% on that, just something I read. Thanks @Tinkickerfor the part numbers, I'll order these tonight and hope that's they do the trick .

 

Cheers all

[Thehunted]

Hi all, just to update. Fitted some new genuine pilot and main jets and she now revs like she should. Still needs a bit of fine tuning and whoever designed that air box needs a good talking too as it's such a poor design. Even with new carb rubbers I've a slight air leak, it's such a wrestle getting it all in properly. So thanks for the good advice all.